Conventionally, there has been provided a power supply device for transforming a relatively low frequency AC voltage such as from a commercial voltage source into a high frequency current. Such power supply device is required to solve a technical problem of restraining input current distortion while maintaining a high input power factor. In order to solve the technical problem, a prior power supply device has been proposed which includes a power factor improving circuit, for example, composed of a step-up chopper to convert an AC voltage into a DC voltage, and an inverter for converting the resulting DC voltage into a high frequency power. That is, various prior power supply devices have been proposed to include the chopper for AC-to-DC conversion and the inverter for DC-to-AC conversion.
However, the chopper requires a relatively large number of components which makes the device bulky with attendant increase in component cost. In view of this, the prior power supply devices have been configured to use the number of components less than the device in which the chopper and the inverter are formed separately from each other so as to achieve a compact and low-cost design.
FIG. 44 illustrates a circuit diagram equivalent to one embodiment of a power supply device disclosed in Japanese laid-open patent publication No. 4-193067. In this circuit configuration, a rectifier DB composed of a diode bridge makes a full-wave rectification of an AC voltage from a voltage source AC and a smoothing capacitor Cdc is connected in series with two diodes D1 and Di across output ends of the rectifier DB so as to convert the AC voltage into a DC voltage. A pair of switching elements Q1 and Q2 is connected across capacitor Cdc. Connected across switching element Q2 is a series circuit of a DC blocking capacitor Cc, an inductor Lrs, and a capacitor Crs across which a discharge lamp is connected as a load L. Switching elements Q1 and Q2 are cooperative with capacitor Cc to form an inverter INV of half-bridge configuration and are driven to turn on and off alternately at a frequency sufficiently higher than that of the voltage source AC. MOSFETs are utilized respectively as the switching elements Q1 and Q2. The inverter thus configured operates to provide a high frequency power from the voltage across capacitor Cdc and supply the resulting high frequency power to the load L through a resonant circuit of capacitor Crs and inductor Lrs. In this configuration, it is made to insert a distortion improving capacitor Cin between the inverter output (connection point of inductor Lrs to capacitor Crs) and a connection point of diodes D1 and Di in order to avoid an increase of an input current distortion for keeping a high input power factor.
Regarding the voltage applied across the load L as the high frequency voltage source, the circuit of FIG. 44 can be recognized to have a series circuit of capacitor Cin and the high frequency voltage source connected across the DC output ends of rectifier DB and to have capacitor Cdc connected across DC output ends of rectifier DB through diode Di. Since the series circuit of capacitor Cin and the high frequency voltage source develops thereacross a voltage which is equal to that of the DC output ends of rectifier DB, capacitor Cin is caused to repeat being charged and discharged in accordance with polarity of voltage developed across the high frequency voltage source. That is, two conditions occur alternately at a high frequency in one of which a charging current flows into capacitor Cin through diode D1 from rectifier DB, and in the other of which voltage across the high frequency voltage is added to voltage across capacitor Cin to apply the resulting voltage to capacitor Cdc. Since the input current from AC voltage source can be supplied at the high frequency through this operation, the input current continuously flows from the AC voltage source for restraining an increase in the input current distortion when a high frequency blocking filter is provided between the AC voltage source and capacitor Cin. Also with this configuration, the voltage across capacitor Cdc is held substantially constant and the voltage amplitude of the high frequency voltage source is held substantially constant such that the input current to capacitor Cin is made approximately proportional to the voltage of the AC voltage source with an attendant increase of the input power factor.
The like operation is also realized in a circuit of FIG. 45 which corresponds to one embodiment disclosed in Japanese laid-open publication No. 5-38161. In this circuit, a load L and a capacitor Crs have their one ends commonly connected to a point between diodes D1 and Di. Also, a capacitor Cin3 is utilized instead of capacitor Cin and is connected across diode Di so as to reduce the increase of input current distortion for keeping the high input power factor.
The above-mentioned prior circuit configurations are generally referred to as a charge-pump type in which capacitor Cin is charged by an input current from the AC voltage source and capacitor Cdc is charged by capacitor Cin. In the power supply device of the charge-pump type, the AC voltage source provides an input of flowing a charging current into capacitor Cdc even the load L consumes no substantial power in the no-load (or light-load) condition, giving an energy surplus between the input and output of the inverter and thereby leaving a problem that capacitor Cdc is caused to develop thereacross excessively increased voltage. With this result, the electronic components particularly the switching elements Q1 and Q2 must be selected to have high dielectric strength, which poses a problem of incurring high component cost.
Japanese Patent laid-open Publication No. 7-288984 discloses another prior power supply device in which feedback means is provided to feedback a portion of a high frequency output from an inverter through a plurality of paths to a smoothing capacitor so as to superimpose the portion of the high frequency output upon a voltage developed across the smoothing capacitor. The device is designed to differentiate the voltages fed-back through the individual paths with each other for reducing crest value of an input current to the device or regulating a current for charging the smoothing capacitor in accordance with the output from the inverter. However, the device necessitates a rather complicated configuration of regulating the frequency of the inverter in order to vary the feedback amount of the voltage from the inverter in anticipation of varying load conditions, for example, loading of a discharge lamp. In fact, the prior device fails to monitor the changing load condition and therefore fails to vary the feedback amount of the voltage in exact correspondence to the changing load condition. Thus, the prior device is not successful for limiting otherwise developed unduly high voltage at the smoothing capacitor over a wide range from a no-load condition, lamp-starting condition, and a normal lighting condition.